Field of the Invention
The present invention relates to an ion bombardment apparatus capable of cleaning a surface of a base material as a pre-treatment of film formation, and a method for cleaning of the surface of the base material using this apparatus.
Description of the Related Art
Conventionally, for the purpose of improving abrasion resistance of a cutting tool and improving a sliding characteristic of a sliding surface of a mechanical part, a hard film (of TiN, TiAlN, CrN, or the like) is formed on a base material (film formation object) by the physical vapor deposition (PVD) method. An apparatus used for forming such a hard film includes a physical vapor deposition apparatus such as an arc ion plating apparatus and a sputtering apparatus.
In such a physical vapor deposition apparatus, in order to improve adhesion of the film to be formed, it is known that a surface of the base material is purified (cleaned) before forming the film. This pre-treatment includes heating cleaning with electron bombardment and a method for cleaning by generating heavy inert gas ions such as argon ions with plasma discharge and irradiating the ions from this plasma to the base material so as to heat the surface (ion bombardment method).
As a technique for performing the pre-treatment of the film formation described above, U.S. Pat. No. 5,294,322 describes an apparatus for cleaning a surface or heating a base material before film formation. In this apparatus, an arc evaporation source including a shutter openable and closable relative to the base material, and an anode independent from this arc evaporation source in a chamber. An inert gas such as an argon gas is introduced into the chamber, argon ions are generated by utilizing arc discharge caused between the arc evaporation source whose front surface is covered by the shutter and the chamber, and the argon ions are irradiated to the surface of the base material having negative potential by applying potential between the anode and the base material, so that the surface of the base material is cleaned.
Japanese Patent No. 4208258 discloses a technique for cleaning a surface of a base material by forming arc discharge (plasma supply source) in a space vertically ranging over treatment height of the base material or this treatment height or more on the inner circumferential side or the outer circumferential side of the base material placed around a vertically center axis in a vacuum chamber, and irradiating argon ions generated therein to the base material to which negative bias voltage is applied.
However, the apparatuses for cleaning the surface of the base material as described above hold the following problems.
In the apparatus of U.S. Pat. No. 5,294,322, a temperature of the shutter for covering the front surface of the arc evaporation source becomes high, and a temperature of the base material also becomes high upon receiving radiation heat thereof. Thus, the apparatus is not easily applied to a base material requiring a treatment at a low temperature such as a quenching material. Since droplets from the arc evaporation source are attached to the shutter, frequent maintenance is required. In addition, since the evaporation source cannot be completely covered by the shutter, there is a problem that some droplets are attached to the base material.
Further, this related art is the technique substantially using a titanium target. However, since the apparatus requires a too large space as a mechanism for only cleaning by ion bombardment, cost thereof is high.
Meanwhile, the apparatus of Japanese Patent No. 4208258 is to charge a gas into a negative electrode chamber separately provided in an upper part of the vacuum chamber so as to generate a difference (pressure gradient) between inner pressure of the negative electrode chamber and inner pressure of the vacuum chamber, and by utilizing this pressure gradient, swiftly jet the gas in the negative electrode chamber into the vacuum chamber from a small opening, so as to generate plasma along the vertically center axis.
However, it is essential to charge the gas into the negative electrode chamber at predetermined pressure in order to generate the pressure gradient. There is a need for severely adjusting a diameter of the small opening in order to swiftly jet the gas into the vacuum chamber. Since a peripheral part of the small opening formed of expensive molybdenum or the like is always exposed to gas jet, the peripheral part is harshly worn away. Thus, an economical burden is increased.
In order to evenly clean the surface of the base material, an increase in size of the apparatus is unavoidable, and a plurality of positive electrodes is placed, so that a system is complicated. Thus, a distance between the surface of the base material and the plasma (that is, a cleaning effect) is not easily maintained constantly.
Further, in a case of cleaning of the surface of the base material using such an apparatus, when pressure of a process gas is high, a distance of potential gradient (sheath) generated around the base material is extended. Therefore, in a case of the base material having a complicated shape (for example having a groove part), the sheath is not generated along the complicated shape part but generated so as to surround the whole base material, so that the gas ions such as Ar ions cannot reach the complicated shape part. As a result, the base material having a complicated shape cannot be uniformly cleaned. Conversely, when the pressure of the process gas is low, there is a problem that a plasma state (state that the ions and electrons are disassociated) is not easily maintained.
In addition, there are not only the problems for cleaning described above but also problematic points for a circuit in these apparatuses (apparatus described in U.S. Pat. No. 5,294,322 in particular). That is, since all the power supplies are connected via the vacuum chamber, the power supplies form a loop via the chamber. When such a loop is formed, a problem that current control is instabilized is generated in a case where GNDs (grounds) actually have different potential from each other or the like.